This disclosure relates generally to acoustic treatments and, more particularly, to acoustic treatments in an unducted area of a geared turbomachine.
Turbomachines, such as gas turbine engines, typically include a fan section, a turbine section, a compressor section, and a combustor section. Turbomachines may employ a geared architecture that allows the fan to spin at a slower rotational speed than the low pressure turbine.
Air moves into the turbomachine through the fan section. Some of this air moves into a core of the turbomachine. The remaining air moves through a bypass flowpath established between a fan cowl and a core engine cowl of the core. The core of the turbomachine extends axially outside the fan cowl. That is, some of the bypass stream is ducted and some of it is unducted.
Acoustic treatments attenuate noise radiating from the turbomachine. These acoustic treatments are traditionally limited to ducted areas of the turbomachine. Extending acoustic treatments to unducted areas is less efficient as a result of the reduced interaction between the propagating noise and the acoustic treatment surface. Moreover, placing the acoustic treatment in unducted areas subject to high Mach number flows (e.g. near a nozzle exit) results in a performance penalty associated with increased drag. In a typical turbofan with a fan pressure ratio greater than 1.5, the high performance penalty associated with the high nozzle Mach number along with the reduced noise benefit makes it impractical to place acoustic treatment on the unducted area past the fan nozzle exit plane.